The Synthesis And Properties Of Organic Small Molecule Photovoltaic Materials Based On Isoindigo

Bulk heterojunction (BHJ) organic solar cells (OSCs) have been a hot topic in the field of solar cells owing to their distinctive characteristics of lightweight, low cost and flexibility. In the past several years, intensive efforts have been poured into studying the polymer BHJ OSCs and the dramatic power conversion efficiency (PCE) of 10.61% has been reported. As an alternative to polymer BHJ OSCs, solution-processed organic small molecules (OSMs) OSCs have received much attention due to their advantages of well-defined chemical structure, easier chemical modification and facility of purification. To date, OSMs-based BHJ devices exhibit outstanding photovoltaic performance with PCEs over 10%. To meet the requirement of commercialization, new organic small molecule photovoltaic materials should be designed and synthesized for high PCEs.Eight D-A-D OSMs were designed and synthesized with isoindigo (ID) as electron-accepting (A) units. According to different electron-donating (D) units, eight molecules can be divided into series A and series B respectively. In series A, triphenylamine (TPA) as D units were linked with ID by benzene, styrene, phenylacrylonitrile and phenylacetylene as bridges, respectively. In series B, carbazole or fluorene as D units were linked with isoindigo by acetylene as bridge or without bridge, respectively.The electron density distribution and energy levels, optical properties, electrochemical properties of eight molecules were investigated by density functional theory calculation, UV-vis absorption spectroscopy and cyclic voltammetry. To investigate photovoltaic properties, solution-processed small molecule BHJ solar cells were fabricated. Based on the above tests, we get the following conclusions:(1) The ID-based molecules possess low-lying HOMO energy levels contributing to high open-circuit voltage (Voc) over 0.9 V. (2) In series B, carbazole-based molecules offer better performance of all aspects than fluorene-based molecules; the introduction of acetylene devotes to the coplanar and adjustment of energy levels. (3) In series A, phenylacrylonitrile as bridge can effectively lower the HOMO energy level and improve the ability of absorption. The devices based on (TPACB)2ID have impressive photovoltaic properties with PCE of 3.57%, Voc of 1.06 V, short-circuit current of 9.46 mA cm-2 and fill factor of 0.35, which is the highest PCE among devices comprising isoindigo molecular semiconductors without additives.